Heat-besistant insulating materials



Patented Apr. 5, 1949 HEAT-RESISTAN T INSULATING MATERIALS Oscar Kenneth Johannson, Corning, N. 2., assignor to Corning Glass Works, Corning, N. Y., a corporation of New York N Drawing.

3 Claims. 1

This invention relates to compositions of matter for use in the production of coatings, films and plastics which are characterized by their ability to withstand elevated temperatures. More particularly, it deals with heat resistant insulating materials and methods of making them.

It is generally recognized that conventional organic films, coatings and plastics are limited in their utility by temperature and that they cannot be used for continuous service at temperatures over about 150 C. This limitation puts a considerable restriction on safe service temperatures of many insulating materials and has particularly put a temperature limit on the design of electrical equipment.

In the construction of electrical apparatus large amounts of electrically insulating material are required. In certain applications, the insulation is applied by winding or shaping to some predetermined form in order to accommodate coils or other electrical conductors. In order to avoid the temperature limitations of typical organic insuiations such as paper, shellac, and cellulosederivative films, designers have usedinorganic spacing materials such as mica sheets, glass fiber cloth and fibrous asbestos. Y

There has long been a need for a flexible electrical insulating material which can be provided as a continuous sheet or tape, which can be used in thin sections and which will withstand relatively elevated temperatures without substantial deterioration in its insulating characteristics. Mica has been employed widely in thin sheets but requires a high proportion of hand labor in utilizing it either as splittings or in the manufacture of laminated insulating materials. Glass filaments and threads, alone or Woven into cloth and tapes, provide insulation which can be handled by winding machinery but losesinsulation resistance when wet due to moisture penetration of the interstitial spaces. Fibrous asbestos is subject to similar objections and is further bulky to use. V

The primary object of this invention is to provide a composition of matter capable of yielding films, coatings and plastics characterized by their resistance to deterioration at elevated temperatures and by their flexibility and toughness at application temperatures.

It isa particular object of the invention to ApplicationFebruary 28, 1945, Serial No. 580,268

, 2 provide a fiexible heat resistant insulating material in the form of a free film which can be slit into wrapping tapes and which will have sufficiently good mechanical properties to permit easy winding on coils, bars, and other conductors by conventional machinery. A further object is to provide such tape or sheet insulation having the plroperty of becoming thermally cementitious, thereby permitting the conversion of wrapped insulation to a sealed, unitary covering over the conductor or coil, or providing a laminating film for use in the production of other insulating constructions.

Another object of the invention is to provide a thin flexible insulating material which may be used in condensers, capacitors and the like at elevated temperatures. A further object is to provide a method of insulating conductors with heat resistant continuous insulation.

Other objects include the provision of articles of manufacture useful in the design of electrical equipment to operate at temperatures above those considered safe for conventional organic insulations.

These and other objects not specifically set forth will become apparent in the detailed description and examples of the invention given below.

I have now found that the above mentioned and related objects can be attained to an unexpected degree by employing compositions of matter comprising certain organo-silicon oxide condensation polymers, commonly referred to as "silicones and solvent soluble cellulose ethers.

In carrying out the invention, the organosilicon oxide product is prepared in a state of condensation at which it is still dispersible in organic solvents. A solution of the said silicone is added to a solution of the cellulose ether in a common solvent to yield a clear oompatiblefilm forming by evaporation to produce coatings or films. Al-

ternately, the composition may be freed of solvent in a mixer of the Banbury type, preferably under reduced pressure, to produce a plastic molding composition. In either case, the solvent free com-' through a thermoplastic stage and is converted to anon-softening heat resistant product.

01 the cellulose ethers, I prefer to use ethylcellulose having from 2.25 to 2.75 ethyl groups per anhydro-glucose unit. Other organo soluble cellulose ethers that can be used are propyl cellulose, butyl cellulose, benzyl cellulose, ethyl benzyl cellulose and the like providing that they are freely and completely soluble in aromatic hydrocarbons.

Cellulose ethers'alone are limited in their service temperatures by' the fact that they become embrittled when heated at temperatures within to C. of their softening point; they tend to carbonize at temperatures above their softening point, and in general, they undergo rapid oxidative degradation at these temperatures. However, compositions comprising the organosilicon oxide polymers of the present invention and commercial grades of ethyl cellulose were found to be unexpectedly stable to heat, even when the ethylcellulose constituted a major part of the composition and the fihn or coating had been subjected to temperatures well above the softening point of the contained ethylcellulose.

When such compositions are heated, they soften and become deformable at temperatures from 100 C. to 200 0., depending upon the resin content and its degree of condensation. Each composition has a temperature threshold above which it softens enough to become heat scalable or flowable under slight pressure. Further heating above this point generally results in conversion of the composition to an infusible, heat resistant state. It is likely that the stabilization of the ethylcellulose by the organo-silicon oxide polymer is due to the fact that the latter enters into combination with the former through some condensation mechanism not fully understood. In any event, the heated compositions are no longer soluble in the same solvents. Further, they are infusible, or at least much higher in their melting point, and are surprisingly more stable at elevated temperatures than would be expected considering their content of products based upon carbon as the essential structural element.

The organo-silicon oxide compositions for use in the film forming materials of this invention are those whichare derived from mixtures of compounds having the general formulae RSiX: and R'zSiYz, respectively. R and R represent the same or different alkyl radicals attached directly to silicon through carbon-silicon linkage and X and Y represent the same or different halogen, alkoxy or other hydrolyzable groups. The hydrolysis product of a compound having the general formula RSiX'; is capable of condensing in three directions as follows:

- ing the general formula R'zSiYz are capable of The hydolysis products of the compounds havcondensing in two directions as follows:

R'gSiY, Manon RzSiO The products obtained by the condensation of the disubstituted silicols whether partially or completely dehydrated are incompatible with cellulose ethers for all practical purposes. At best they will tolerate only from 5 to 10% of ethyl cellulose and the latter must be of the high ethoxy type.

When mixtures of compounds having the general formulae RSiX1 and R'zSiYz are hydrolyzed and the hydrolysis products dehydrated, copolymers are formed comprising both 'mono-alkyl and dialkyl substituted silicon units joined together through slloxane linkages as described in the copending application of James Franklin Hyde, Serial No. 318,373, filed February 9, 1940, now Patent 2,386,466.

spond substantially to the formulae RSiOs/z and RsSiO, respectively, the units being joined together through silicon-oxygen linkages.

I have found that these condensation products of certain mixtures of RSiX1 and R'zSiYz are compatible with ethyl cellulose in all proportions and will yield extremely useful film forming compositions when mixed with the cellulose ethers. in the condensation product should not be in ex-' cess of 35 mole per cent. The condensation copolymers of this invention are also limited to those which are still soluble enough in aromatic hydrocarbons to give a gel-free viscous solution. Such copolymers on removal of solvent are capable of condensing further on baking at elevated temperatures to a substantially insoluble and infusible state. When dissolved in a common solvent with ethyl cellulose they yield upon evaporation of the solvent a free, self-supporting, coherent film. The film so obtained is flexible, water-impervious, and possesses desirable electrical properties. In general, I prefer to limit the amount of ethyl cellulose employed to about 60% by weight and to use at least 15%. The resulting films are not only flexible and self-supportin but also have a relatively high thermal stability.

The hydrolysis of mixtures of the compounds RSiX. and R'zSiYz is preferably carried out under such conditions as to substantially incorporate all the RzSiO units formed in the copolymer. The conditions are those which tend to slow down the rate of condensation of the mono-alkyl silicols in order to promote the intercondensation of these silicols with the dialkyl silicols. The use of a solvent is one of such conditions and the use of alkoxy silanes rather than halogenated silanes is another. In the latter connection it is sometimes preferable to use a mono-a1kyltriethoxysilane with a dialkylsilicon dihalide in order to obtain substantially the same rate of hydrolysis and condensation between the two compounds of different degrees of substitution.

When the'hydrolyzable group X or Y is an alkoxy radical, the condensation copolymcrs of this invention include intermediate hydrolysis and condensation products which may still contain appreciable amounts of unhydrolyzed alkoxy groups, that is, up to about one such group per 50 silicon atoms. When the hydrolyzable group is halogen, it is necessary to remove all halogen completely to avoid development of free acid on heating the film composition and its consequent embrittling effects on the cellulose ether.

These copolymers cousist essentially of structural units which corre- The proportion of R2Si0 units present with water.

reference should be had to the following examples.

Example 1 To 53.5 g. (0.3 mole) of methyltriethoxysilane and 14.8 g. (0.1 mole) of dimethyldiethoxysilane,

15 g. of 12.5% HCl was added dropwise at room temperature) The solution was heated to reflux for ten minutes. An excess of water was added to precipitate the copolymer. Theresin was dissolved in ether and was washed first with concentrated HCl and then with water. The process was repeated, this time washing the ether solution to neutrality. Ether was then removed under reduced pressure. tacky after air-drying at'room temperature for forty-eight hours. Set to a weak gel occurred after one hour heating at 160 'C. With such propertiesit would be difficult to form a free film of this material which could be handled without breaking. Mixed with of its weight of high viscosity ethyl cellulose, films could be obtained which air-dried in less than ten minutes. Such films, on bending double, creased but did not crack. However, these films dissolved completely in toluene and had a low softening temperature. On baking, set occurred, resulting in a lower solubility in toluene. After one to four hours at 160 (1., the films swelled to some extent in this solvent but remained intact. However, the films after the four hour bake were still suiiiciently flexible to be doubled without cracking. In fact;

The resin was stillnot dry tack-free in 24 hours. soluble in toluene after such air-drying.

A mixture of the resin and high-ethoxy Ethocel, 10% in the latter, was homogeneous- It was still tacky and soluble in toluene after 24 hours aindrying. After baking one hour at 160 C., it was cloudy; on continuing the heating for four hours, the cloudiness had disappeared in part. The film now did not fuse below 317 C., and was flexible although somewhat weak.

0n baking one hour at 200 C., the film was clear and amber-colored. It was flexible, insoluble in toluene, and did not melt below 317 C. The point of interest is that the mixture was compatible and gave homogeneous films on air-drying but that separation occurred on baking for one hour at 160" C. As the length of time of baking was increased this mixture was apparently again becoming homogeneous. In-

crease of the Ethocel content to 33% gave films which remained clear during the bake.

I Example 3 be obtained. A mixture of 74.1 g. of dimethylafter 18 hours at 160 C., the films in 1 to 2 mil thicknesses broke on bending over a mandrel 1.3 mm. in diameter, but were not broken on bending over mandrels of greater diameter.

After a two hour bake at 160 (3., the power factor, at 1 kilocycle, of such film was 0.16%, when dry, and 0.27% after twenty-fours hours in water. Some flow was noted when the films were rapidly heated to 160 C. This could be eliminated by raising the temperature of the films from that of the room to 195 C., over a period of five and one-half hours.

With mixtures of the copolymer, 50% in ethyl cellulose, films could be cast from a 75:25 toluene-etlianol solution which air-dried in less than five minutes. These dissolved rapidly in toluene. After a two hour bake, the films swelled in toluene but did not dissolve. Power factors of films baked in this way were 0.20% and 0.79% respectively, before and after a twenty-four hour immersion in water.

Example 2 To 178.3 g. (1 mole) of methyltriethoxysilane.

was precipitated by adding a large volume of i water, and was then taken up in 200 ml. of ether. The ether solution was washed once, to free it in part from alcohol. To the solution, 50 ml. of cone. HCl was added and the mixture was stirred 30 minutes. An excess of water was added, the ether layer separated and washed once The treatment. with HCl was repeated. The ether solution was washed to neutrality. Ether was removed and the copolymer was heated thirty minutes at 115 C. and 15 mm.

The viscosity at 25" C. of a 50% solution of the resin in toluene was 4.9 cs. tent of the resin was 2.2% and the silicon content 39.7%. The resin in 1 to 2 mil films did The hydroxyl condiethoxysilane and 178.3 g. of methyltriethoxysilane was added dropwise to 200 g. of 0.2% HCl at 0 C. The product was taken. up in ml. of ether and was washed to neutrality. Ether was removed at room temperature with the pressure first at 15 mm. and, finally, less than 0.1 mm. The product contained 11.15% OH.

Mixtures of this copolymer were prepared with Dow high-ethoxy and medium-ethoxy Ethocel and with Hercules low viscosity ethyl cellulose.

' With all the types mentioned, mixtures were prepared which were 10, 20, and 30% in ethyl cellulose. All these mixtures were much slower in airdrying than were corresponding mixtures with the previous copolymer. The mixtures'20% in ethyl cellulose were still tacky after twenty-four hours; the mixtures 30% in ethyl cellulose required twelve hours to set.

However, on baking at 0., very rapid set occurs. After one hour at this temperature, films will not soften below 395v C. The product was homogeneous at all times during the baking.

Example 4 A solution of 98.1 g. (0.6 mole) of ethylsilicon trichloride and 31.4 g. (0.2 mole) of diethylsillcon dichloride in 200 m1. of ether was added dropwise to 200 g. of ice and. water with stirring. The solution was washed to neutrality and ether was removedunder reduced pressure. The copolymer was compatible with 10% medium ethoxy Ethocel before and after baking. It was compatiblewith high-ethoxy Ethocel'in mixtures containing 10, 25 and'50% of the latter. The 10% mixtures are slow in air drying; the 25% mixture set up in ten minutes and the 50% mixture in five minutes. The compositions set to hard films on baking.

I claim:

1. A composition of matter comprising 40 to 85 percent by weight of resinous copolymeric organo silicon oxide condensation product'consisting of the structural units which correspond substantially to the general formulae RS103 and R'zSiO respectively, said units being in such It was completely bon to give a gel-free viscous solution. N mber 2. A composition of matter in accordance with claim 1 in which R and P are methyl radicals.

3. A composition of matter in accordance with 10 2,253,220 claim 1 in which R and R are ethyl radicals. OSCAR KENNETH J OHANNSON.

REFERENCES CITED The following references are of record in the file of this patent: c

UNITED STATES PATENTS Name Date Bonniksen Feb. 11, 1936 Rochow Oct. 7, 1941 Rochow Oct. 7, 1941 

